For decades, vacuum tubes were incorporated into almost every aspect of electronic equipment, including audio amplifiers. With the advent of transistors followed by integrated circuits, vacuum tubes fell into disuse such that few, if any, modern televisions, radios, and similar electronic devices use vacuum tubes. However, a substantial group of “true” audiophiles consider the sound quality and performance of transistors and integrated circuits in audio equipment (e.g., stereo amplifiers) to be unsatisfactory because the sound from such modern amplifies tends to be cold and sterile. An increasing niche market exists for audio amplifiers having vacuum tubes in the power output stages.
Two major methods of power stage operation are used to implement vacuum tube amplifiers. The first major method of audio amplification is implemented as a single-ended amplifier. In a single-ended amplifier, a single power vacuum tube is provided for the final power output stage of each audio channel. In some embodiments, two or more vacuum tubes may be connected in parallel; however, the multiple tubes operate in unison as a single tube. Although the discussion herein is directed to a single vacuum tubes, it should be understood that the discussion encompasses multiple tubes connected in parallel. Because only a single vacuum tube is provided, the tube must handle (e.g., amplify) an entire input signal from a lowest input amplitude to a highest input amplitude. This operation is referred to as Class-A operation. The single vacuum tube is biased so that the tube conducts plate current throughout the entire 360 degrees of the AC cycle of the input signal and so that the tube is maintained in a highly linear range of operation at all times.
The second major method of audio amplification is implemented as a push-pull amplifier. In a push-pull amplifier, two tubes (or two sets of parallel tubes) work together; however, each tube operates during alternate AC half cycles of the input signal with respect to the other tube. Accordingly, each of the tubes in the push-pull amplifier performs half of the work of the single tube in the single-ended system. In the push-pull system, the two tubes are fed with an AC input signal; however, the signal fed to one tube is 180 degrees out of phase with the signal fed to the other tube. The two signals are otherwise identical and may be derived from a phase inverter. The outputs (e.g., the plates) of the two tubes in the push-pull amplifier are summed, for example, in a center tapped output power transformer. Because of the phasing of the input signals to the two tubes, the first tube, for example, increases conduction during one half cycle while the second tube decreases conduction. Then, during the next half cycle, the second tube increases conduction while the first tube decreases conduction. Effectively, one tube pushes current while the other tube pulls current into and out of the output power transformer.
In general, a push-pull amplifier provides better efficiency than a single-ended amplifier, however, many advocates of single-ended amplifiers assert that single-ended amplifiers provide a better sound but at a cost of reduced efficiency. Furthermore, singled-ended amplifiers are limited by the amount of power that can be provided by a singled tube without overheating the tube.
Known single-ended amplifiers are typically implemented with one of three circuit topologies. FIG. 1, for example, illustrates a circuit 100 that implements a conventional topology wherein a triode vacuum tube 110 has a grid terminal 112, a cathode terminal 114 and a plate terminal 116. The grid terminal is fed with an input signal to be amplified from an AC input signal source 118. The cathode terminal is connected to a circuit ground reference 124 via a cathode-biasing resistor 120 and a parallel-connected first AC-coupling capacitor 122. The idle current flowing through the biasing resistor biases the cathode to a positive voltage with respect to the grid terminal such that the grid is at a negative voltage with respect to the cathode. The cathode of the vacuum tube in the illustrated topology is biased to a desired idle operating point for the plate voltage and the plate current.
In FIG. 1, the plate terminal 116 of the triode vacuum tube 110 is connected to a first terminal 134 of a primary winding 132 of a power output transformer 130. A second terminal 136 of the primary winding is connected to the output (VPLATE+) of a plate voltage source 140, which is referenced to the circuit ground reference 124. The second terminal of the primary winding is also connected to the circuit ground reference by a second AC-coupling capacitor 142 that allows AC signals to bypass the plate voltage source. A secondary winding 150 of the power output transformer is connected to an audio transducer (e.g., a loudspeaker) 152 or to another load.
The input signal applied to the grid terminal 112 of the vacuum tube 110 causes the plate voltage to vary about the idle operating point to cause an AC current to flow through the primary winding 132 and through the first and second AC-coupling capacitors 122, 142. The varying AC current through the primary winding induces a secondary voltage in the secondary winding 150, which causes current to flow through the loudspeaker (or other load) 152.
The circuit 100 of FIG. 1 operates well to amplify the input signal; however, all of the AC current flowing through the primary winding 132 must also flow through the vacuum tube 110 throughout the entire cycle of the input signal. Because the DC plate voltage and the DC idle current must also flow through the primary winding, the power output transformer 130 may require a gapped core to avoid saturation of the core.
A second known topology is a parallel feed (“parafeed”) topology, which is represented by a circuit 200 in FIG. 2. The parallel feed topology includes a triode vacuum tube 210 having a grid terminal 212, a cathode terminal 214 and a plate terminal 216. The grid terminal is fed with an input signal to be amplified from an AC input signal source 218. The cathode terminal is connected to a circuit ground reference 224 via a cathode-biasing resistor 220 and a parallel-connected first AC-coupling capacitor 222. The idle current flowing through the biasing resistor biases the cathode to a positive voltage with respect to the grid terminal such that the grid is at a negative voltage with respect to the cathode. The cathode of the vacuum tube in the illustrated topology is biased to a desired idle operating point for the plate voltage and the plate current.
The plate terminal 216 of the vacuum tube 210 is connected to a first terminal 234 of a primary winding 232 of a power output transformer 230. A second terminal 236 of the primary winding is connected to the circuit ground reference 224 via a second AC-coupling capacitor 240. A secondary winding 250 of the power output transformer is connected to an audio transducer (e.g., a loudspeaker) 252 or to another load. A common connection node 260 between the plate terminal of the vacuum tube and the first terminal of the primary winding is connected to a first terminal 264 of an inductor 262. A second terminal 266 of the inductor is connected to a plate voltage (VPLATE+) source 270, which is referenced to the circuit ground reference. Accordingly, the DC idle current for the vacuum tube is provided to the plate terminal via the inductor. The only current flowing through the primary winding of the power output transformer is an AC current resulting from changes in the plate voltage caused by the AC input signal applied to the grid terminal. Accordingly, the power output transformer can be smaller and simpler (e.g., no gap in the core).
In the circuit 200 of FIG. 2 implementing the parafeed topology, the inductor 260 inhibits AC current from passing through the plate voltage source 270. As in the circuit 100 of FIG. 1 implementing the conventional topology, the vacuum tube 200 of FIG. 2 in the parafeed topology must handle the AC current flowing through the primary winding throughout the entirety of each AC cycle.
A third known topology is a constant current source topology represented by a circuit 300 in FIG. 3. The constant current source topology includes a triode vacuum tube 310 having a grid terminal 312, a cathode terminal 314 and a plate terminal 316. The grid terminal is fed with an input signal to be amplified from an AC signal input source 318 via a first AC-coupling capacitor 320.
Unlike the biased cathodes of the two previously described circuits 100, 200, the cathode terminal 314 in the circuit 300 of FIG. 3 is connected directly to a circuit ground reference 330. The grid terminal 312 is biased to an idle grid voltage via a grid bias resistor 332 connected to the output of a grid bias voltage source 334. The grid bias voltage source is referenced to the circuit ground reference. As illustrated, the grid bias voltage source produces a negative voltage (e.g., having a magnitude of 45 volts) with respect to the circuit ground reference to establish an idle condition for the plate voltage (e.g., approximately 300 volts for a configuration where the maximum plate voltage is approximately 600 volts).
The plate terminal 316 of the vacuum tube 310 is connected to a first terminal 344 of the primary winding 342 of a power output transformer 340. A second terminal 346 of the primary winding is connected to the circuit ground reference 330 via a second AC-coupling capacitor 348. A secondary winding 350 of the power output transformer is connected to an audio transducer (e.g., a loudspeaker) 352 or to another load.
A common connection node 360 between the plate terminal 316 of the vacuum tube 310 and the first terminal 344 of the primary winding 342 of the power output transformer 340 is connected to an output terminal 372 of a constant current source 370. A second terminal 374 of the constant current source is connected to a plate voltage (VPLATE+) source 380, which is referenced to the circuit ground reference 330. A constant DC idle current for the vacuum tube is provided to the plate terminal from the constant current source. The only current flowing through the primary winding of the power output transformer is an AC current resulting from changes in the plate voltage caused by the AC input signal applied to the grid terminal. Accordingly, the power output transformer can be smaller and simpler (e.g., no gap in the core).
In the circuit 300 of FIG. 3 implementing the constant current source topology, the constant current source 370 provides a current corresponding to the desired idle current of the vacuum tube 310 at all times. When the plate voltage increases in response to a more positive input signal applied to the grid terminal 312, a portion of the current output by the constant current source is provided to the primary winding 342 of the power output transformer 340, which reduces the current flow through the vacuum tube. On the other hand, when the plate voltage decreases, the plate current increases to sink both the full magnitude of the output of the constant current source as well as the current from the primary winding. Accordingly, the total power consumption of the vacuum tube includes power generated by the additional current from the constant current source flowing through the vacuum tube. The additional power consumed by the vacuum tube reduces the amount of amplification provided by the vacuum tube because the total power must be maintained below a safe power level for the vacuum tube.